The Sex Life of Yeast




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Yeast Basics > Lifecycle > Yeast Mating

Okay, so calling it sex may be overstating things a bit, but yeast can (and do!) mate, and this mating is an incredibly useful tool for yeast geneticists. So, here's the scoop…

Laboratory yeast strains can be cultured and maintained as haploid cells (like eggs or sperm) or diploid cells (as in somatic tissues), carrying one or two versions of the yeast genome, respectively. Haploid S. cerevisiae come in two mating types, a and a, and each mating type secretes its corresponding pheromone, a small peptide that tells neighboring cells which mating type it is (this works much better than those pheromone colognes and perfumes that were so trendy a few years back!). When a particular haploid cell senses pheromone from the opposite mating type, it will prepare to mate by forming a mating projection as shown in the figure below. This mating projection is commonly called a "shmoo," because it causes the yeast cells to take on a shape similar to that of Al Capp's cartoon character, shmoo. If shmooed cells of opposite mating types meet, mating can occur, resulting in the fusion of the two cells and formation of a dumbbell-shaped zygote. Ultimately, the two nuclei in this zygote will fuse (a process called karyogamy) to create a diploid cell that can be propagated as such in the lab. During vegetative growth, diploid cells reproduce by budding as described on the previous page.

Just as our bodies use meiosis to create haploid eggs or sperm, yeast use meiosis to go from the diploid to the haploid state. A key trigger for initiating meiosis in yeast is nitrogen starvation. Upon completion of the meiosis program, a diploid yeast cell will have been converted to four haploid spores, two of mating type a and two of mating type a. These spores are surrounded by a tough coat called an ascus, and this packet of spores is extremely resistant to environmental stresses. In the wild, these spores would germinate when conditions improve, and the resulting yeast would mate with each other to give rise to diploid budding yeast. In the laboratory, however, scientists take advantage of spore formation by dissecting these siblings away from each other and tracking traits in the haploid progeny of diploid yeast.


Next:  Yeast In Culture

For a more thorough review of yeast mating and meiosis try...


Champion M.D. and R.S. Hawley (2002) Playing for half the deck: the molecular biology of meiosis. Nat Cell Biol. 4 Suppl:s50-6.


Dohlman, H.G. and J.W. Thorner (2001) Regulation of G protein-initiated signal transduction in yeast: paradigms and principles. Annu Rev Biochem. 70:703-54.


Engelbrecht, J. (2003) Cell signaling in yeast sporulation. Biochem Biophys Res Commun. 306:325-8.


Herskowitz, I. (1995) MAP kinase pathways in yeast: for mating and more. Cell. 80:187-97.


Neiman, A.M. (2005) Ascospore formation in the yeast Saccharomyces cerevisiae. Microbiol Mol Biol Rev. 69:565-84.



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